The whole idea about kids sharing roles came from England where they have very strict child labor laws. That was something that ended up happening here when shows like Les Miserables transfered to Broadway from London and had two actors play Gavroche and two actors split the roles of young Cosette and Young Eponine.
If you look at a show like Big that had a huge cast full of child actors, none of them were double cast at all. Not even the kids who played Young Josh or Billy.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll